Silver is chemically the most reactive element among the noble metals and tarnishes readily on exposure to sulfur bearing atmospheres. Discoloration, generally termed tarnishing, is caused by a silver oxidation process in which sulfide is formed. Food such as onions, mustard and eggs which contain organic sulfur compounds are also known to tarnish silver. See Singh et al., "Silver Tarnishing and its Prevention--A Review" Anti-corrosion Methods and Materials, v. 30 (July 1983) pp. 4-8.
Silver tarnishing is also known to occur when an oxygen bleaching agent used in detergent compositions oxidizes the silver to silver oxide. This oxidation process causes surface blackening of the silver leaving undesirable tarnishing of silverware when machine dishwashed.
Conventional detergents, particularly automatic machine dishwashing detergents, are generally formulated with chlorine bleaching agents in a high alkaline pH range. During washing, certain chlorine bleaches (e.g., chloroisocyanurate) react to form isocyanuric acid and thus do not greatly effect silver discolorization.
Detergent compositions are, however, increasingly being based on peroxygen bleaching agents and are being formulated to be milder to produce more environmentally friendly products. The problem of tarnishing of silver and silver plated articles has thus become more severe.